362 QUEENSLAND AGRICULTURAL JOURNAL. [1 Nov., 1898. 
Hor this reason, the natural production of these soluble ingredients in 
tropical soil, corresponds very closely to the quantities of plant-food ingredients 
removed by the various crops, and, as a general rule, the necessity of restoring 
these materials by artificial means arises sooner in temperate regions than in 
the tropics. 
On the other hand, the restoration, by natural agencies, of plant-food 
ingredients consumes more time in temperate regions, on account of their 
slower transformation into a soluble state; the soil is relatively more rapidly 
exhausted in the tropics, and only by artificially supplying it with the 
necessary plant-food ingredients can such rapid exhaustion be prevented. 
Moreover, complaints about “ worn-out” soils are more frequently heard 
in the tropics than in temperate regions, because, in the latter, rational 
cultivation is in its prime, whilst in the tropics it is, with very few exceptions, 
still in its infaney; in other words, the system of continuous cropping is still 
the leading principle of agriculture in the tropics. 
On account of the immense tracts of new land available in the tropics, this 
system could be followed, to alarge extent, without much damage, for as soon 
as a soil became worn out cultivation could be begun anew on a virgin soil. 
For various reasons, this system cannot be so easily put into practice atthe 
present time, and the planters in tropical and sub-tropical regions are accord- 
ingly compelled to resort to a rational system of fertilisation, and to supply 
their fields artificially with the necessary plant-food ingredients that have been 
withdrawn by continuous cropping for many years. As, however, a system of 
‘rational cultivation las been but recently begun, agricultural experiments, 
which are so necessary to its success, and which have been so useful in temperate 
regions, are hardly as yet in an advanced stage. 
The system of making agricultural experiments ‘owes its development to 
the recognition of the fact that only through such can we discover the right 
methods to be pursued in bringing back soils to their former state of fertility. 
Therefore, the results obtained from such a system of rational cultivation 
in tropical regions, although yet few in number, are of great importance, not 
only in encouraging planters to follow new methods, but also in inducing them 
to cultivate anew old soils that were formerly regarded as unproductive. 
PART I. 
GENERAL REMARKS ON TROPICAL MANURING. 
Every planter knows that the plant-food ingredients, even in the best 
soils, will disappear in the course of time. The plants will become less vigorous 
in their growth, and the yields will consequently diminish in proportion, even 
in spite of the fact that they may have received the best of care otherwise, 
This result is due not only to deterioration in the physical condition of the 
soil, but also to the loss of certain plant-food ingredients, the presence of which 
is absolutely essential to the growth of crops. Plants require a large number 
of elements for their proper nutrition, which are gathered from the ar, the 
soil, and the water in the soil, by means of roots and leaves. The substances 
taken from the air and water are already present in sufficient quantities, and 
besides this the earth contains sufficient supplies of those mineral substances 
which the plants only require in smaller quantities. On the other hand, the 
supply of the most important plant-food ingredients—i.c., nitrogen, phosphoric 
acid, and potash—is limited in most soils, as these elements have been princi- 
pally removed by continuous and vigorous cropping. ‘The three ingredients 
just mentioned must therefore be artificially supplied sooner or later. In 
addition to the three substances mentioned, there is also lime and sometimes 
magnesia. These, however, need only be supplied in certain soils which are 
deficient in those ingredients. 
The object of fertilisation is to supply to the soil, in an assimilable state, 
those ingredients in which it is deficient. Fertilisers may be divided into 
several groups, according to their composition and to the effect they produce, 
